The present invention relates to an apparatus that controls tilting of an axle in an industrial vehicle such as a forklift. More particularly, the present invention pertains to an axle tilt control apparatus for locking an axle, which is tiltably supported by a vehicle's body frame, against tilting.
A typical industrial vehicle such as a forklift includes a rear axle that tilts relative to the body frame. The rear axle tilts such that all the wheels always contact the ground even if the vehicle runs over bumps and dips on the road surface. This structure improves comfort and stability of the vehicle.
However, when a carried object is heavy and is elevated to a relatively high position or when the vehicle turns at a high speed, tilting of the axle may destabilize the vehicle. Thus, an apparatus for locking the rear axle to the frame has been proposed.
Japanese Unexamined Patent Publication No. 58-167214 discloses an industrial vehicle having such an apparatus. The middle of the vehicle's rear axle is coupled to the body frame to be tiltable relative to the body frame. The ends of the rear axle are coupled to the body frame by a pair of single-movement hydraulic cylinders, respectively. The hydraulic cylinders each have a piston rod and an oil chamber that is connected to a common electromagnetic control valve by a line. Each piston rod is extended by supplying oil to the corresponding oil chamber. The control valve is moved between a open position to connect the oil chambers with each other and a closed position to disconnect the oil chambers from each other. Connecting the oil chambers with each other permits oil in the chambers to flow back and forth. As the piston rod of one of the hydraulic cylinders is extended, the piston rod of the other cylinder is retracted. Accordingly, the axle is tilted relative to the body frame. When the oil chambers are disconnected from each other, oil in the chambers cannot flow between the chambers. Thus, the piston rods are fixed. As a result, the axle is locked to the frame.
The hydraulic cylinders of the Japanese publication are single-movement type. In such a hydraulic cylinder, the pressure of the oil in the oil chamber prevents the piston rod from retracting when oil flow from the oil chamber is restricted. In this state, however, the oil pressure does not prevent the piston rod from extending. Application of single-movement hydraulic cylinders to an axle locking apparatus therefore raises the following problems. If one of the hydraulic cylinders malfunctions, for example, if a piston seal ring is damaged, the hydraulic cylinder allows its piston rod to extend and retract even if the electromagnetic control valve is at the closed position. On the other hand, the other hydraulic cylinder, which is functioning normally, does not restrict the extension of its piston rod. The axle is therefore not locked relative to the body frame.